1. Field of the Invention
This invention relates generally to a device for checking the roundness of a cylindrical workpiece, more specifically to a holding fixture for rotatably supporting a cylindrical segment such as a missile tube portion so that the critical dimensions of the cylindrical segment may be inspected.
2. Description of the Prior Art
The manufacture of high technology devices frequently imposes the requirement that in the construction of such devices great care be taken to ensure adherence to critical tolerance requirements. Failure to closely inspect components during the manufacturing process may result in rejection of assemblies, costly reworking of components, delays in production, or critical failures in operation of the device. Nowhere are such critical tolerance requirements more important than in aerospace and defense applications such as missile and rocket assemblies.
As an example, missiles may be constructed from several segments which are welded together. One such segment is a spacer segment, which is a cylindrical tube separating the surface control section of a missile from other sections containing the guidance, propulsion, and warhead portions of the missile. The spacer section is constructed from flat stock by rolling the metal into a tube and welding the adjacent edges together, thus forming a cylindrical tube. Since the spacer section is made of fairly thin metal to keep weight to a minimum, when it is welded to an adjoining cylindrical segment it is critical that both segments be concentric and have the same diameter.
Typically, both segments are machined prior to welding so that when adjacent segments abut, a V-shaped channel is formed into which the welded bead may be laid. After the segments are welded together, the weld is then machined flat with the inner surfaces of the segments.
It can therefore be seen that if one or both of the segments are not concentric or of the required diameter, the segments will not abut properly, resulting in the machining operation removing a portion of the wall thickness of one of the segments. Thus, the structural integrity of the assembly will be degraded, and the assembly must then be rejected.
It is thus apparent that the cylindrical segments used in the missile housing must adhere to critical tolerances, and in order to avoid defective assemblies careful inspection of the housing components such as the spacer section must take place before the sections are welded together. Inspection of the cylindrical housing segments, which are typically around 8 inches in diameter, has in the past been performed on an inspection surface plate, which is made of perfectly flat and polished rock. Using a dial indicator mounted on a stand resting on the surface plate, the inspector had to rotate the housing segment by hand while reading the dial indicator. It is evident that such an operation is rather difficult and time consuming, as well as presenting the possibility of somewhat less than complete accuracy.
A number of devices exist in the general subject area of gaging or measuring the outer perimeter of circular or curved elements. One such device is illustrated in U.S. Pat. No. 3,751,812 to Meyer. The Meyer device, which is used to measure the ovality of an out-of-round device, is unsuitable for checking the concentricity of a cylindrical tube. This limitation exists because the Meyer device does not support the workpiece on a plurality of support points having tangents which are orthogonal axes, thereby alowing the workpiece to be rotated with respect to these fixed support points and measured for concentricity with respect to the axes. Rather, Meyer uses a pivotably mounted jaw and a single fixed support point, thereby allowing substantially out-of-round articles to be supported, and rendering impossible a measurement of concentricity, or even a measurement of diameter sufficiently accurate to ensure the adherence to critical dimensional tolerances required by the missile spacer section described above.
Another such device is described in U.S.S.R. Inventor Certificate No. 485,302, which is a device for checking the mean diameter of the races of ball bearings. This device drives a dial indicator through contacts at two orthogonal tangential positions on the ball bearing to obtain a mean diameter of the race of the bearing being measured. Unfortunately, such an approach is useless for the present application since a first surface defect at one location being measured combined with a second surface defect at the second location being simultaneously measured may result in the dial indicator presenting absolutely no indication of either defect. It is thus possible that a workpiece having a perfect elliptical shape would present an identical reading on the dial indicator with a workpiece having a perfectly round shape.
A variety of other devices for measuring circular or cylindrical workpieces is known in the prior art, as exemplified by U.S. Pat. Nos. 2,501,130 of Kuppersmith, 2,668,362 of P. W. Johnson, 2,696,675 of Case, 2,770,050 of S. G. Johnson, 3,065,549 of Antoszewski et al, 3,104,472 of Eppler, 3,169,323 of Hold and 3,871,106 of Hegenbart, and by German Pat. No. 2,829,000. All of the identified devices are of relatively intricate construction often requiring elaborate set-up procedures which, unless rigorously adhered to, inherently lead to errors in the measurements made by the device. Many of the known devices are adjustable to accommodate workpieces of differing dimensions, thereby further complicating the construction and the set-up procedures which are required.
The need served by the present invention involves quickly and reliably taking the measurement of concentricity about the circumference of a thin-walled cylindrical workpiece of one particular diameter. The measurement is to be made at each end of the cylindrical workpiece. The workpieces may be of different lengths, but the diameter is the same. For efficiency in operation, the holding fixture must permit a workpiece to be quickly and easily located within the fixture at a precise reference position, rapidly rotated therein while the measurements of concentricity are made, and as quickly and easily removed to make way for the next workpiece. The holding fixture of the invention avoids the above-described disadvantages of the Meyer device and the Russian device, while allowing the cylindrical spacer section to be quickly, easily and conveniently rotated and measured to determine whether or not the part is concentric, and of the desired diameter. It is also desirable that the device be provided without exhorbitant cost, so the holding fixture must be of relatively simple and economic construction.